describe the role of gibberellin in the germination of barley (see 16.3.4)

Control and Coordination in Plants

The Role of Gibberellin in Barley Germination (16.3.4)

Barley seeds are like tiny time‑bombs. Inside each seed lies an embryo that is ready to grow, but it is wrapped in a protective coat and surrounded by stored food (mainly starch). Gibberellin (GA) is the hormone that acts as the “starter button” for the germination process. Think of GA as a 🔑 that unlocks the seed’s growth engine.

When the seed absorbs water, GA is produced in the embryo and travels to the endosperm. There it triggers the synthesis of hydrolytic enzymes such as α‑amylase. These enzymes break down starch into sugars, providing the energy needed for the embryo to grow and push through the seed coat. The sequence can be summarised as:

1. Water uptake → activation of GA synthesis
2. GA travels to endosperm → up‑regulation of α‑amylase genes
3. α‑Amylase breaks down starch → sugars released
4. Embryo uses sugars → radicle emerges, seedling grows

Mathematically, the rate of starch breakdown can be described by a first‑order reaction: $$ \frac{d[S]}{dt} = -k[S] $$ where $[S]$ is the starch concentration and $k$ is the rate constant influenced by GA levels.

Exam Tip: When asked about the role of gibberellin, remember to mention:

• GA is produced in the embryo.
• It induces α‑amylase production in the endosperm.
• α‑Amylase converts starch to sugars, providing energy for growth.
• Without GA, the seed remains dormant.

🌱 Analogy: Imagine the seed as a spaceship. GA is the launch code that activates the engines (α‑amylase). The engines burn the stored fuel (starch) to produce the thrust (sugars) that propels the spaceship (embryo) out of its protective shell (seed coat). Without the launch code, the spaceship remains on the launch pad, dormant and waiting.